The present invention relates to prefabricated structural elements comprising a substrate based on cured and dried plaster. More particularly, the invention relates to prefabricated structural elements of the tile, panel and board types. With regard to boards, those relevant to the present invention have a relatively small average thickness, for example ranging from one to a few centimeters, in a preferred direction or dimension, with a limited mass per unit area, for example about 10 kilograms per m2. By way of preferred but nonlimiting example, reference will be made to prefabricated elements of the board type, in the knowledge that at the present time there are, for example, plaster fiberboards in which the plaster contains homogeneously dispersed fibers throughout the mass, these boards being commonly called xe2x80x9cGFBsxe2x80x9d, and plasterboards coated with cardboard or with mats or sheets of mineral fibers (for example glass fibers) on both their faces.
The present application corresponds to French Patent Application No. 99/01885 of Dec. 2, 1999, the text of which is incorporated in that of the present patent application, as required.
The invention applies to other prefabricated elements made of plaster, for example to tiles, or parallelepipedal elements having grooves or slots and tenons on their side edges.
The invention relates to prefabricated structural elements, such as those defined above, having improved water resistance.
The prefabricated structural elements as defined above, having the ability to withstand water, are used either inside buildings, for example in humid rooms such as bathrooms, in order to lay plastic coverings or tiles thereon, or on the outside of buildings, for example for the covering of facades, as long as, nevertheless, these prefabricated elements are coated with an impermeable protective skin.
The expression xe2x80x9cwater resistantxe2x80x9d should be understood to mean the ability of a prefabricated structural element as defined above to limit the uptake of water by the plaster substrate, while still retaining the dimensional stability and mechanical integrity of the structural element in question.
Depending on the countries, this water resistance is codified or regulated by specific standards. Thus, reference may be made to the standards mentioned below, depending on the element manufactured:
plaster tile: NF P72-210-1 (April 1994);
plasterboard: ASTM 630/630M-96a;
board with a glass mat: ASTM C1178/1178M (96).
The ASTM 630/630M-96a standard requires in particular that, when such a structural element is immersed in water for two hours, the water uptake by the plaster substrate is less than 5% and the water absorption on the surface (called the Cobb equivalent) is less than 160 g/m2.
For the purposes of making a structural element, as defined above, water-repellent in accordance with document JP-A-0 730 411, it is known to use a water-repellent agent comprising at least one silicone compound in aqueous and basic medium and to homogeneously incorporate the latter into the substrate of the element in question.
According to JP-A-0 733 0411, the silicone compound is emulsified with water. An oxide or hydroxide of a metal from group IIa, for example calcium, is then added to the emulsion. The additive thus obtained is then incorporated into the dry matter in order to obtain the plaster substrate in the wet state, which substrate is then formed and then dried.
In practice, it appears that the aforementioned composition has the advantage of causing the silicone compound to cure or crosslink, thereby enhancing the water-repellency of the latter; however, this is accompanied by significant evolution of hydrogen, which is particularly dangerous.
Document JP-A-80109251 discloses a water-repellent composition for cement, comprising a silicone compound, the nature of which is not specified, and bentonite as mineral additive.
Document SU-A-1409607 discloses a water-repellent composition for plaster, comprising a sodium methyl siliconate or ethyl siliconate, and bentonite as mineral additive.
In practice, such a composition is able to swell, causing rheological problems of the substrate in the slurry state.
It appears in practice to be difficult to meet the abovementioned standards using contents of silicone compound of less than 1% by weight, said compound being, moreover, very costly.
This water repellency is achieved by using any process comprising, in general, at least the following steps:
(a) homogeneously mixing a dry material, comprising mostly at least one hydratable calcium sulfate, the abovementioned water-repellent agent, and water and preforming said substrate in the wet state;
(b) drying the preformed substrate in order to obtain said substrate formed in the solid and dry state.
With such a process, it is difficult to control the amount of water-repellent agent incorporated into the plaster substrate, for example in the form of a silicone oil, and therefore the potential of the water-repellent agent is not exploited.
The object of the present invention is therefore to better control the water repellency or a structural element using a water-repellent agent comprising at least one silicone compound, in particular to increase its water repellency within the cured plaster.
The object of the present invention is also to improve the introduction and mixing of the water-repellent agent in the process defined above.
In accordance with the present invention, it has been discovered that the addition, as a homogeneous mix, of a mineral additive comprising at least one mineral component having a synergistic water-repellency affinity with the silicone compound to the dry matter from which the substrate is preformed in the liquid state allowed the water repellency of the cured plaster to be increased to a significant extent.
The expression xe2x80x9csynergistic water-repellency affinityxe2x80x9d should be understood to mean the property whereby, by adding the mineral additive according to the present invention, and all other things being equal, the water repellency of the cured-plaster substrate is increased by at least 20%, in Cobb equivalent, according to the ASTM 630/630M-96a standard, and/or, for the same water resistance, the amount of water-repellent agent required is reduced by at least 20%.
Preferably, but nonlimitingly, the mineral component does not react with the silicone compound in the sense that, after two hours, mixing 0.1 g of said silicone compound with the mineral component in aqueous medium may release an amount of hydrogen of less than or equal to 6 ml under standard temperature and pressure conditions.
The evolution of hydrogen is measured according to the following operating method.
An apparatus identical to that described in the NFT 77-150 standard on page 6 is used. The principle consists in bringing the mineral additive and the silicone compound into contact with each other, in aqueous medium in the reactor, in the same proportions by weight as those in the homogeneous mix defined above, in the slurry state, that is to say with the mixing water. The evolution of hydrogen is monitored over time, at a controlled temperature of 23xc2x0 C. The volume of hydrogen evolved after two hours is noted.
A mineral component that hardly reacts at all with the silicone compound is one which produces a parasitic evolution of hydrogen, after two hours, of less than or equal to 6 ml under standard temperature and pressure conditions.
A mineral additive according to the invention therefore makes it possible to increase the water resistance of a structural element according to the invention and/or to reduce the amount of water-repellent agent required for a predetermined level of water resistance.
According to the present invention, with regard to a crystalline mineral component containing a hydroxyl functional group which is nonionizable in an aqueous medium, one possible explanation (among other hypotheses) for the observed synergy, supported by the experimental protocol described below, is the formation of at least one hydrogen bond, in neutral or acid aqueous medium, between, on one side, the hydrogen of a nonionizable hydroxyl functional group of the mineral component and, on the other side the oxygen of the silicone compound.
In other words, according to the invention, the mineral component adsorbs the silicone compound. Furthermore, the homogeneous mixing of the mineral additive into the plaster substrate allows the homogeneous distribution and the fixation of the silicone compound.
During the curing, the hydroxyl ions naturally present in the aqueous medium may bring about three-dimensional crosslinking of the silicone compound, this being manifested by an improvement in the water-repellency properties of the cured plaster over time.
Consequently, the adsorption of the silicone compound on the mineral component makes it possible to control its reactivity, that is to say to delay the latter until the moment of curing the plaster.
Preferably, according to the invention, the reactive silicone compound is a linear, cyclized or branched macromolecular water-repellent compound containing polysiloxane units, each of which is chosen from the group consisting of the groups (R1R2R3SiO1/2), (R1R2SiO) and (R2SiO3/2), with:
R1=H, Cl, or any other halogen, or an alkoxy
R2=a branched or unbranched phenyl or alkyl
R3=a branched or unbranched phenyl or alkyl.
By way of example, the silicone is an alkyl hydrogenopolysiloxane, such as methyl hydrogenopolysiloxane, in the form of an oligomer (several tens or more of siloxane units), in the form of an oil or of an aqueous emulsion.
Preferably, according to the invention, the reactive mineral component is a crystalline mineral material which contains a hydroxyl functional group which is nonionizable in aqueous medium, for example a clayey material and/or a crystalline silica.
As an example of such a crystalline mineral material, mention may be made of a clayey material, preferably one which does not swell in aqueous medium (unlike pentonite), comprising especially an aluminosilicate, for example an illite and/or a kaolin.
A mineral additive according to the invention optionally comprises an inert mineral component.
In accordance with the invention, the mineral additive is inert with respect to calcium sulfate, initially in powder form. It comprises, for example, a clayey material, as mentioned above.
By virtue of the invention, and when a papermaking-type process is used, especially by filtration of a slurry, to manufacture GFB boards based on cellulose fibers, it has been found that the incorporation of a mineral additive comprising, for example, a clayey material made it possible to retain the water-repellent agent within the plaster substrate preformed in the wet state. This makes it possible to increase, for the same content of water-repellent agent, the water resistance of the GFB boards manufactured by such a process, and in particular to limit the amount of this agent in the recycled water of hydration.
The present invention also has the following secondary features:
the composition by weight of the mineral additive in the dry matter, in combination with the amount and nature of the water-repellent agent, is tailored to give the cured-plaster structural element a water resistance which meets the aforementioned standards, and in particular the ASTM 630/630M-96a standard, for a so-called GFB board;
the reactive silicone compound represents a proportion by weight of the total dry matter introduced of at least 0.1% and preferably between 0.1 and 1%;
the reinforcing fibers, for example cellulose fibers, are distributed in the cured-plaster substrate; the structural element is, for example, a so-called GFB fiberboard;
the element has the shape of a board and the two faces of the cured-plaster substrate are coated with two sheets of cellulose fibers, for example with cardboard;
the mineral additive has a particle size distribution such that the particles having a size of less than or equal to 63 xcexcm represent a proportion by weight of at least 85% of said mineral additive; preferably, the mineral additive has a loss on ignition at 900xc2x0 C. of less than 30%; by way of example, the mineral additive has a loss on ignition of less than 30%; by way of example, the mineral additive represents a proportion of the total weight of the ingredients, excluding water of hydration, of at least 5% and preferably at most 15%;
the hydratable calcium sulfate is obtained from a natural gypsum or from a sulfogypsum, the latter coming from the desulfurization of gases from thermal power stations;
the reactive silicone compound may be introduced during the manufacture of the structural element, in the form of an emulsified composition of a silicone in water;
by way of example, in combination, the mineral additive comprises a clayey material, a crystalline silica aid, optionally, an inert mineral supplement compatible with the clayey material and dispersible in the cured-plaster substrate;
the composition by weight of the mineral additive is tailored, in conjunction with the water-repellent agent, also to give the structural element, used in wall partitions, a fire resistance which meets the so-called xe2x80x9cGKFxe2x80x9d standards according to DIN 18180 and 4102 (part IV);
the inert mineral supplement, if it is present in the mineral additive, is, for example, dolomite and the mineral additive then comprises, in approximately equal proportions by weight, the clayey material (including the crystalline silica) and the inert mineral supplement; this particular mineral additive makes it possible, with the water-repellent agent, to obtain boards which are both fire resistant and water resistant and which in particular meet both the GKF standards and the aforementioned water-resistance standards, whatever the process used to obtain the plasterboards or to obtain so-called xe2x80x9cGFBxe2x80x9d fiberboards;
reinforcing fibers, preferably mineral fibers compatible with the hydratable calcium sulfate, for example glass fibers, are also distributed in the cured-plaster substrate in a proportion by weight with respect to the total of the dry matter introduced of less than 1% when it is desired, in particular, to improve the fire resistance of the boards;
by way of another example, the mineral additive comprises an amorphous silica and/or diatomaceous earth and/or ferrite and/or a zeolite;
more generally, the mineral additive according to the invention comprises a mineral component having an affinity with respect to the silicone compound, finely ground (for example xe2x89xa61 xcexcm), taken from clays; smectites are preferred, then illites and kaolinites;
the structural element according to the invention has a shape chosen from the group consisting of tiles, panels and boards.
The present invention also relates to a water-repellent composition comprising as homogeneous mix, a water-repellent agent comprising a silicone compound and a mineral additive comprising at least one mineral component having a synergistic water-repellency affinity with said silicone compound, as defined above.
The invention also relates to the use of this composition within a plaster substrate of a prefabricated structural element.
The invention also relates to the use of a mineral additive to increase the water repellency of a water-repellent agent comprising a silicone compound. According to the invention, the mineral additive comprises at least one mineral component having a synergistic water-repellency affinity with said silicone compound, as defined above.
The invention also relates to a process for manufacturing a structural element comprising a substrate based on cured plaster, in which said substrate is obtained by at least the following steps:
a) dry matter comprising mostly at least one hydratable calcium sulfate, a water-repellent agent comprising at least one silicone compound and water of hydration are mixed homogeneously and said substrate is preformed in the wet state;
b) the preformed substrate is dried in order to obtain the substrate formed in the solid and dry state.
According to the invention, during step (a), a mineral additive, comprising at least one mineral component having a synergistic water-repellency affinity with said silicone compound, as defined above, is also mixed.
Preferably, the ingredients defined in step (a) are mixed directly and together, in order to obtain the substrate in the wet and preformed state.
Such a process may have the following variants:
the water-repellent agent is in liquid form, particularly in the form of an oil, or of a homogeneous aqueous suspension, while the mineral additive is a powder in divided form;
the water-repellent agent is mixed homogeneously with the dry matter comprising the mineral additive, before said dry matter is mixed with the water of hydration, in order to obtain the substrate preformed in the wet state;
the water-repellent agent is mixed homogeneously with the water of hydration, before the latter is mixed with the dry matter comprising the mineral additive, in order to obtain the substrate preformed in the wet state;
the water-repellent agent and the mineral additive are mixed homogeneously in order to obtain a powder imbibed with oil on the surface, which powder is then incorporated into the dry matter and/or the mixing water before they are homogeneously mixed, in order to obtain the substrate preformed in the wet state;
the water-repellent agent, in the form of an oil or an aqueous emulsion, is homogeneously dispersed in the mixing water, whereas the mineral additive is distributed homogeneously in the dry matter.
According to the present invention, it has been discovered that if the homogeneous mixing of the above-defined ingredients, prior to the preforming of the substrate in the wet state, also includes a mineral additive as defined above, and in particular a clayey material, then this mineral additive is able to act as a retention agent for the reactive silicone compound within the plaster-based substrate.
The solution according to the invention has the major advantage of remaining compatible with the various processes known at the present time for manufacturing the structural elements considered by the present invention, for example:
(a) by casting a plaster-based slurry between two sheets of cellulose fibers, in the form of cardboard, or between two mats, wovens or sheets of mineral fibers;
(b) by filtering a slurry based on cellulose fibers and on plaster, using a papermaking-type process, in order to obtain so-called GFB fiberboards;
(c) by so-called semi-wet or semi-dry compression, depending on the water content (GFB fiberboards).
Applied to each of these processes, the invention makes it possible to increase the efficiency of incorporation of the water-repellent agent within the plaster substrate. For example, for a process involving the filtration of a slurry, of the papermaking type, the invention makes it possible to keep the water-repellent agent within the cake obtained by filtration, rather than concentrate it in the filtrate, despite recycling the water of hydration.
Throughout the description below, the term xe2x80x9ccured plasterxe2x80x9d or xe2x80x9cplasterxe2x80x9d will be reserved for calcium sulfate dihydrate (CaSO4.2H2O) or regenerated gypsum.
The term xe2x80x9chydratable calcium sulfatexe2x80x9d should be understood to mean a mineral compound or a composition consisting of or comprising an anhydrous calcium sulfate (anhydrite II or III), a calcium sulfate hemihydrate (CaSO4.1/2H2O), whether in the crystallinexe2x80x94xcex1 or xcex2xe2x80x94form of the latter.
The term xe2x80x9csilicone compoundxe2x80x9d should be understood to mean any compound capable of being obtained by the polymerization of a silicone-type monomer unit, whether this is, for example, a chlorosilane, a siliconate, an, alkoxysilane or a silanol.
The term xe2x80x9ccellulose fibersxe2x80x9d should be understood to mean discrete elements such as fibers, filaments and chips, based on natural, regenerated, recycled or modified cellulose; preferably, the cellulose fibers in question are those generally used in the composition of papers and cardboards.
The term xe2x80x9cmineral fibersxe2x80x9d should be understood to mean inorganic fibers, for example glass fibers, which may or may not be treated in order to remain chemically stable and inert once they have been incorporated into the cured-plaster substrate of the prefabricated structural elements considered by the present invention.
As is known, dolomite is a mixed calcium and magnesium carbonate.
Throughout the present invention, unless otherwise specified, all the compositions are expressed with respect to the total weight of the dry matter introduced, including the water-repellent agent. These compositions may be easily transposed with respect to the final weight of the prefabricated, finished and dry structural element, taking into account the water of rehydration of the calcium sulfate.
According to the present invention, the mineral additive used does not develop by itself, in aqueous medium, an alkaline pH. This excludes as mineral component any material capable of releasing, in aqueous medium, OHxe2x88x92 ions, for example:
a) a mineral base, such as calcium dihydroxide; or
b) a metal silicate, for example containing no hydroxyl ions, for example a calcium silicate, such as, that sold by Lafarge under the reference CAP 52,5 CP2; or
c) an aluminate, for example a calcium aluminate, such as that sold by Lafarge under the reference SECAR 80.
In this regard, reference will be made to FIG. 1, which shows the evolution of hydrogen (at atmospheric pressure and at 23xc2x0 C.) for three mixes according to the following representation:
the solid line corresponds to a mix comprising 0.1 g of Ca(OH)2+0.1 g of MHPS (BS94 from Wacker)+65 ml of water;
the dotted line corresponds to a mix comprising 1.72 g of a calcium silicate (the abovementioned commercial ref.)+0.1 g of MHPS (BS94 from Wacker)+65 ml of water;
the dot-dash line corresponds to a mix comprising 1.72 g of a calcium aluminate (the abovementioned commercial ref.)+0.1 g of MHPS (BS94 from Wacker)+65 ml of water.
The volume expressed in ml is plotted on the x-axis and the contacting time, in hours, is plotted on the y-axis.
This is because it is sufficient to mix any of the aforementioned mineral components (a) to (c) with a reactive silicone compound, for example methyl hydrogenopolysiloxane (abbreviated to MHPS), in aqueous medium, to immediately observe a violent and abundant evolution of hydrogen.
As indicated previously, a reactive mineral component of the bentonite type appears to be excluded for the manufacture of certain structural elements comprising a substrate based on cured plaster, such as plasterboards.